We propose studies of the role of a new class of protein kinases in the regulation of a well studied in vivo growth model, rat liver regeneration. These kinases phosphorylate tyrosine residues and have recently been discovered by those investigating viral transformation (e.g. the src kinase of Rous Sarcoma virus) and growth factor action (Epidermal Groth Factor dependent - EGF receptor phosphorylation in A-431 cells). Nucleic acid hybridization studies have indicated that the viral associated tyrosine residue kinases may be constituents of normal cells. Because tyrosinephosphorylation is correlated with neoplastic growth and growth factor action it has been suggested that these kinases may participate in growth regulation. To date the majority of experiments have been confined to neoplastic cells in culture in which normal regulation is abrogated or in which the growth factor, EGF, does not stimulate growth (A-431 cells). The present proposal details studies of EGF-dependent tyrosine residue phogphorylation in rat liver regeneration. Our data indicate that EGF binding and EGF-dependent kinase activity are altered in vivo after partial hepatectomy. Other changes in protein kinase activity during regeneration are noted. For example, partial hepatectomy results in increases in total membrane phosphorylation, in several specific phosphoproteins and in phosphotyrosine content. The experiments described will focus on the purification and biochemical characterization of the hepatic EGF receptor. Data indicate that hepatic EGF-dependent phosphorylation differs in several respects from that observed in the tyrosine residue kinases are induced during hepatic growth states including regeneration, fetal growth, and neoplastic transformation. The consequences of EGF receptor phosphorylation and internalization will be studied in intact hepatocytes. The study of EGF-dependent phosphorylation, and tyrosine residue kinase induction in a well characterized in vivo growth model should provide insight into the function of this new class of kinases.